function sendAjax(url) {
    return new Promise((resolve, reject) => {
        //1.创建对象
        const xhr = new XMLHttpRequest();
        xhr.responseType = 'json';
        //2. 初始化
        xhr.open('GET', url);
        //3.发送
        xhr.send();
        //4. 处理响应结果
        xhr.onreadystatechange = function () {
            if (xhr.readystate === 4) {
                //判断响应状态码2xx
                if (xhr.status >= 200 && xhr.status < 300) {
                    //控制台输出响应体
                    resolve(xhr.response);
                } else {
                    //控制台输出响应状态码
                    reject(xhr.status);
                }
            }
        }
    })
};
sendAjax('https://api.apiopen.top/getJoke')
    .then(value => {
        console.log(value);
    },
        reason => {
            console.log(reason);
        })

// btn.addEventListener('click', sendAjax.then(value => {
//     console.log(value);
// },
//     reason => {
//         console.log(reason);
//     })
// );